1. Field of the Invention
The present invention relates to a ball bearing for a strain wave gearing.
2. Description of Related Art
A strain wave gearing is conventionally known which includes an annular circular spline 80, an annular flex spline 99, and a rotary member 89 as depicted in FIG. 9 (see Japanese Patent Application Publication No. S60-143244 (JP S60-143244 A)). The circular spline 80 has internal teeth 81. The flex spline 99 is provided inside the circular spline 80 and has external teeth 86 that mesh with the internal teeth 81. The rotary member 89 is provided inside the flex spline 99. In the strain wave gearing, the number of the external teeth 86 is set smaller than the number of the internal teeth 81. The rotary member 89 has a cam 91 and a ball bearing 90. The ball bearing 90 is externally fitted over the cam 91, with the flex spline 99 externally fitted over the ball bearing 90. The cam 91 has an elliptic shape. Thus, the ball bearing 90 and the flex spline 99 lying outside the cam 91 are deflected to have an elliptic shape, allowing the external teeth 86 of the flex spline 99 to partially mesh with the internal teeth 81 of the circular spline 80. In other words, the flex spline 99 deflected to have an elliptic shape meshes with the circular spline 80 at the portion of a major axis of the flex spline 99 deflected to have an elliptic shape and separates from the circular spline 80 at the portion of a minor axis of the flex spline 99.
The cam 91 is rotated to allow major axis positions (the positions where the flex spline 99 meshes with the internal teeth 81) of the elliptic flex spline 99 to be moved with respect to the circular spline 80. In conjunction with this movement, the flex spline 99 can be rotated with the teeth of the flex spline 99 partially meshing with the circular spline 80.
The ball bearing 90 provided outside the cam 91 having an elliptic shape has an outer ring 98, an inner ring 92, a plurality of balls 96, and an annular cage 97. The flex spline 99 is externally fitted over the outer ring 98. The inner ring 92 is externally fitted over the cam 91. The balls 96 are arranged in an annular space 95 formed between the outer ring 98 and the inner ring 92. The cage 97 holds the balls 96.
FIG. 10A is a transverse sectional view depicting the ball bearing 90 and a peripheral portion thereof. The cage 97 has a ring portion 97a and a plurality of cage bars 97b. The cage bars 97b extend from the ring portion 97a in an axial direction. The cage 97 is known as what is called a snap cage. In the cage 97, pockets 94 that hold the respective balls 96 are each located between the adjacent cage bars 97b in a circumferential direction.
The outer ring 98, the inner ring 92, and the cage 97 in the ball bearing 90each have a perfect round shape before the ball bearing 90 is attached to the cam 91. When the ball bearing 90 is attached to the cam 91, the outer ring 98 and the inner ring 92 are elastically deformed into an elliptic shape. In contrast, the cage 97 acts to maintain the perfect round shape. In such a strain wave gearing, possible backlash of the ball bearing 90 needs to be suppressed in order to prevent the elliptic shape of the flex spline 99 from being impaired. For this purpose, the ball bearing 90 for a strain wave gearing has more balls than standard ball bearings.
FIG. 10B is a diagram depicting the ball 96 and a part of the cage 97 as viewed in a direction parallel to an axis of the ball bearing 90. In the cage 97 of the conventional ball bearing 90 for a strain wave gearing, the pocket 94 is shaped along a spherical surface. The spherical surface is set to be slightly larger in radius than the ball 96. Thus, a clearance formed between the ball 96 and the pocket 94 is very small. This configuration allows the cage 97 to be positioned in a radial direction and the axial direction when the balls 96 and the pockets 94 come into contact with one another.
In the ball bearing 90 which is used for the strain wave gearing depicted in FIG. 9 and which is externally fitted over the elliptic cam 91, the balls 96 are placed in an elliptic arrangement in conjunction with the deformation of the inner ring 92 into an elliptic shape. Therefore, in the conventional ball bearing 90, the balls 96 are placed in an elliptic arrangement with respect to the cage 97 that acts to maintain a perfect round shape. In particular, at portions S1 (see FIG. 9) corresponding to the major axis of the ellipse, the clearance between the ball 96 and the pocket 94 (see FIG. 10B) is partially lost. Thus, the cage 97 may be deformed to cause a local stress. Furthermore, the cam 91 rotates to repeatedly cause such a stress. Moreover, the revolution speed of the ball 96 varies between two positions S3 and S4 across the portion S1 of the major axis (see FIG. 9). Consequently, possible advancement and delay of the balls 96 may cause each of the balls 96 to collide against the cage bar 97b (pocket 94), leading to an excessive stress on the cage 97.
Thus, the clearance formed between the ball 96 and the pocket 94 (see FIG. 10B) may be set to be larger in order to reduce the stress generated on the cage 97 due to the relationship between the balls 96 in the elliptic arrangement and the cage 97 (pockets 94). However, in this case, the cage 97 is unstably positioned in the radial direction and the axial direction. Thus, in particular, while the major axis direction of the ball bearing 90 deformed into an elliptic shape coincides with a vertical direction (see FIG. 9), the balls 96 at positions in a minor axis direction may push the cage bars 97b of the cage 97, and the cage 97 may wobble in conjunction with rotation. As a result, the cage 97 may be damaged in a short period of time, or vibration or noise may occur.